Porous PTFE membranes are generally produced as follows. A mixture obtained by mixing a PTFE fine powder and a liquid lubricant serving as an extrusion aid is extrusion-molded, and the resulting molded body is rolled to form a PTFE sheet. The liquid lubricant is removed from the PTFE sheet, and then the resulting PTFE sheet, from which the liquid lubricant has been removed, is stretched to make the sheet porous. Thus, a porous PTFE membrane is produced (see, for example, Patent Literature 1). The porous PTFE membrane thus obtained has a porous structure of nodes and fibrils, as is well known.
When the porous PTFE membrane is used as a collection layer of an air filter medium, it is usually bonded to an air-permeable support member such as a nonwoven fabric to provide the required strength to the membrane. The porous PTFE membrane and the air-permeable support member are bonded together by heat lamination, lamination using an adhesive (adhesive lamination), or the like.
Pressure loss and collection efficiency are important properties of an air filter medium. However, it is difficult to achieve a good balance between these two properties because one of them tends to deteriorate when the other improves. A PF value is often used as a measure for evaluating whether the pressure loss and the collection efficiency are well balanced or not. The PF value is calculated by the following Equation (1). The higher the PF value of an air filter medium, the higher the performance thereof. In Equation (1), the permeability PT and the collection efficiency CE have a relationship expressed by the following Equation (2). PL is the pressure loss.PF value={−log(PT(%)/100)/(PL(mmH2O)}×100  (1)PT(%)=100−CE (%)  (2)
An air filter medium requires a porous PTFE membrane having a high PF value to achieve a good balance between pressure loss and collection efficiency. In order to produce a porous PTFE membrane having a high PF value, various improvements have been proposed for each step of porous PTFE membrane production methods.
For example, Patent Literature 1 proposes that in the step of stretching a PTFE sheet to make the sheet porous, after stretching in the longitudinal direction (MD direction), the PTFE sheet be stretched in the transverse direction (TD direction) at a high stretching speed (paragraph 0023). Patent Literature 2 proposes that in the step of mixing a PTFE fine powder and a liquid lubricant, a large amount of liquid lubricant be added (paragraphs 0053 to 0055).